1. Field of the Invention
The present invention relates to a storage device; in particular, to a hybrid density memory storage device.
2. Description of Related Art
Non-volatile memory (also known as Electrically Independent memory) is used to store data, commonly applied to storage devices (e.g. as memory card, thumb drive with USB interface, solid state drive etc.) and digital devices to be employed in conjunction therewith for data storage. Flash memory provides advantages of high storage density, low power consumption, effective access efficiency and reasonable price, thus becoming nowadays the mainstream of current non-volatile memory devices.
The memory represents the stored bit values by voltage values conserved in memory cells. In traditional memory storage technology, a bit data as one (1) or zero (0) is record by means of high level or zero level of the voltage value conserved in one memory cell. As the technologies in semi-conductor processes advance, it is possible to further divide the level values conserved within a memory cell, so that more different voltage values can be stored in one memory cell, thus storing more bit values. For example, if a memory cell can retain four different level values, it may store two-bit data. The industry refers the traditional memory manufacture technology for storing one single bit data as the Single-Level-Cell (SLC) process, and memory fabricated thereby as low density memory; memory manufacture technology for storing multi-bit data as the Multi-Level-Cell (MLC) process, and memory fabricated thereby as high density memory.
Compared with low density memory, the data storage capacity provided within a unit area of high density memory increases multifold, offering desirable advantages in terms of significantly increased storage capacity and decreased cost, but it requires longer times for data read/write and erase actions; besides, the MLC process also results in significant reduction in endurance count, collaterally affecting hardware performance such as data access rate and life span in high density memory storage.
The features of high density memory lie in high storage capacity, low cost, low data access rate and low endurance count; contrarily, the advantages of low density memory exist in high erase endurance, high data access rate, low storage capacity and low cost. Since each type of memory respectively demonstrates features thereof, the industry therefore has devoted in the development and application of hybrid density memory. The so-called hybrid density memory comprises both high density memory and low density memory at the same time. Exploiting the advantages of both types of memory to enhance storage performance, has now become a major research subject aggressively promoted by the relevant industries.
Refer now to FIGS. 1A and 1B, which show storage management diagrams for hybrid density memory in the prior art. FIG. 1A illustrates the arrangement of a logical block 80 in a file, and FIG. 1B shows the arrangement of a physical block 82 in the file. As shown in FIGS. 1A and 1B, such a technology stores the individual physical locations for the spare area or control data area 800 and user data area 805 of the file respectively into the low density memory block 820 and high density memory block 825. The reason for this arrangement is that the control data area 800 registering file control information has stronger requirement on access rate; hence, it is allocated in the low density memory storage block 820. However, in such a technology, file update will perform erase actions on corresponding memory blocks 820, 825 respectively for the control data area 800 and the user data area 805 at the same time, thus the high density memory is very likely to achieve its endurance count earlier, causing that the high density memory loses its function before the storage effect of the low density memory being fully employed, resulting in early failure of the memory and wastes of storage resources.
The defects of the above-mentioned method can be improved by means of wear-leveling management technology for hybrid density memory. Said wear-leveling management is to generate weights based on the inverse ratio of endurance counts in the high and low density memories to adjust the erase count recorded for each storage block, then to allocate the physical location of the file according to adjusted erase count, such that the high density memory and low density memory inside the storage device may reach their respective endurance count simultaneously as much as possible, so as to fully exploit the available storage resources. Meanwhile, file allocation management is based on the attribute (e.g. extension name) of the file to assign the physical location of the file to the low density memory or high density memory, in order to store the files requiring higher data access rate or frequently used to the low density memory providing high endurance count. In this file allocation management method, if the storage device can not correctly identify the file system or file attribute, it is impossible to correctly allocate the file to the appropriate memory, thus the objective of storage performance improvement may not be reached.
The defects of the above-mentioned method can be solved by categorizing frequently used data of the file system as hot data, building a hot list inside the storage device, recording the logical location of the hot data, and assigning the physical location of the hot data to the low density memory, so as to fully exploit the life span of low density memory and acquire high data access rate. Although said method can exploit the life span of the low density memory and utilize the high data access rate provided by the low density memory, it can not, however, provide the effect of extending life span of storage device by reducing the erase count of the non-volatile memory. In view of this factor, the inventors propose the present invention with an expectation to accelerate system operation speed and reduce erase count in the non-volatile memory, so as to extend the life span of the storage device.